Embedded Systems

Modern Forth

Stephen Pelc is managing director of MicroProcessor Engineering Ltd. He can be contacted at www.mpeforth.com

Engineering is the art of making what you want from things you can get. --Jerry Avins

I am a design chauvinist. I believe that good design is magical and not to be lightly tinkered with. The difference between a great design and a lousy one is in the meshing of the thousand details that either fit or don't, and the spirit of the passionate intellect that has tied them together, or tried. That's why programming (or buying software) on the basis of "lists of features" is a doomed and misguided effort. The features can be thrown together, as in a garbage can, or carefully laid together and interwoven in elegant unification, as in APL, or the Forth language, or the game of chess.
--Ted Nelson

We know about as much about software quality problems as they knew about the Black Plague in the 1600s. We've seen the victims' agonies and helped burn the corpses. We don't know what causes it; we don't really know if there is only one disease. We just suffer -- and keep pouring our sewage into our water supply.
--Tom Van Vleck

There is increasing realisation that there are no magic bullets for software development. Interactive languages returned to fashion in the web developer world, which also saw a return to multi-language programming -- use each language for what it's good at. I'm going to look at one of these interactive languages, Forth, and show what people are doing with it, and why it's a powerful tool.

Modern Forth systems are not like their ancestors. The introduction of the ANS/ISO Forth standard in 1994 separated implementation from the language. One result was a proliferation of native-code compiling systems that preserved the traditional Forth interactivity while providing the performance of batch-compiled languages. It is the combination of performance and interactivity that makes modern Forth systems so attractive.

What is Forth?

Forth is a member of the class of interactive extensible languages. Interactive means that you can type a command at any time and it will be executed. Extensible means that there's no difference between functions, procedures and subroutines (called "words" in Forth parlance) that you defined and the ones that the system provides. Forth is an untyped language whose primary data item is a cell. A cell is the size of an item on the stacks, normally 16, 32, or 64 bits.

Underlying all languages, there's an execution model or virtual machine (VM). C and the other languages have one and so does Forth. Forth is rare among languages in exposing the VM to the programmer. The VM consists of a CPU, two stacks, and main memory; see Figure 1. The stacks are conceptually not part of main memory. When I refer to "the stack" I mean the data stack.

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Figure 1: C and Forth VMs

The two stacks are called the data stack and the return stack. The data stack holds arguments to words, return values, and transient data. The return stack holds return addresses and can also be used for temporary storage. There are several consequences of having two stacks, the most important of which are:

return addresses do not get in the way of the data stack, so words can return any number of results.

Items on the stack do not have names -- they are anonymous.

Because items on the stack are anonymous, you sometimes have indulge in stack juggling using words like DUP and SWAP to manipulate the stack. In the spirit of the marketing principle "advertise your worst feature", the anonymous stack leads, in part, to Forth's well-deserved reputation for small applications. Because stack juggling requires thought, you improve productivity by reducing thought. Many small words reduce the thought required by splitting the job into manageable chunks. A side effect of this is that you reuse code at a much finer grain in Forth culture than in most others. The ease of code reuse at this level leads to small applications. Factoring at this level means that access to many performance-critcal routines is through small well-defined interfaces. A consequence is that you can make major changes easily. For example, an embedded system can move from EEPROM data storage to a file system very quickly. Forth is a very agile language.

For those who are unfamiliar with Forth, the Forth text interpreter is a little odd. It deals with whitespace delimited tokens. These tokens are looked up in a dictionary. If found they are either executed or compiled. If a token is not found, its is considered to be a number. If number conversion fails, an error occurs. That's all! A consequence of this is that a valid Forth word name can contain any non-whitespace characters. Because there's a lot of interactive testing, many commonly used words are short. The common strange-looking ones are @ (fetch), ! (store) and . (integer print). The collection of word names is called the "dictionary," which can be split into vocabularies (namespaces). For more about the Forth interpreter, see JForth: Implementing Forth in Java by Craig A. Lindley.

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